Phase and Amplitude Control for Optical Fiber Output

ABSTRACT

A method for shaping an output light beam from an optical fiber by controlling the phase and amplitude of the beam by producing beam shaping elements on an exit facet of the optical fiber by direct surface texturing of the exit facet, where a controlled phase difference is achieved across the fiber cross-section over a predefined pattern. The optical fiber can be a single mode fiber or a multi-mode fiber. Either a binary or a complex phase difference can be achieved. Also disclosed is the related system for shaping an output light beam from an optical fiber.

PRIORITY CLAIM

The present application is a non-provisional application claiming thebenefit of U.S. Provisional Application No. 61/786,656, filed on Mar.15, 2013 by Jasbinder S. Sanghera et al., entitled “Phase and AmplitudeControl for Optical Fiber Output,” the entire contents of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to optical fiber outputs and,more specifically, to controlling the phase and amplitude of a lightbeam profile exiting an optical fiber.

Description of the Prior Art

A typical optical system will transmit, reflect, refract or otherwisemodify the propagation of light or its salient properties such as phase,amplitude or polarization. In particular, an optical fiber will presentat the cross-section of the output aperture a beam of lightcharacterized by a certain amplitude (intensity) and phase distribution.The very familiar situation is that of the light propagation through asingle-mode fiber which will have at the output a profile close to thatof a Gaussian beam. The intensity is highest at the center and then itdecreases as radius increases. The Gaussian beams are important becausethey maintain a Gaussian intensity profile at any location along thebeam axis, even after passing through lenses (ignoring lensaberrations). The phase profile of such a beam is also very simple,usually linear or quadratic (described by a polynomial). The quadraticcase is important as it is implying convergence or divergence of thebeam (change in the beam radius).

There are however many situations when a Gaussian beam is not desirable.Particle trapping and ultra high-resolution fluorescence microscopy areachieved using beams that have a ring or doughnut shape (no light in thecenter). Flat top beams, where the intensity is constant over most ofthe cross-section, are also of interest when uniform illumination andefficient focusing are required such as in material laser processing.Most of the work is done in bulk, with light beams manipulated by macrooptics (gratings, phase plates etc.).

Beam shaping can be implemented through different techniques: use ofapertures, use of a combination of various optical elements, such asmicro-lens arrays, or through manipulation of the near field whichresults in the desired changes in the far field. This last method,requiring modification in the near field of the beam phase rather thanamplitude, is easy to implement. It can be achieved by placing a phasemask in the beam path. It also provides the desired profile with minimalloss in total energy. In very few cases direct beam manipulation wasperformed at the output of an optical fiber.

Beam shaping has been researched intensively and a variety of patentshave provided a multitude of approaches. For example, U.S. Pat. No.8,031,414 (2011), U.S. Pat. No. 8,016,449 (2011), and U.S. Pat. No.7,593,615 (2010) provide for instructive reading with respect to variousmeans of beam shaping (all covering refractive methods using externallenses, diffusers, waveguides or other optical elements). Prior artdiscussing the idea of creating a phase mask-like structure directly onthe fiber end is extremely limited. Existing approaches requiredeposition of photosensitive material on the fiber end, material inwhich the surface structure is to be created.

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems are overcome in the present invention whichprovides a method for shaping an output light beam from an optical fiberby controlling the phase and amplitude of the beam by producing beamshaping elements on an exit facet of the optical fiber by direct surfacetexturing of the exit facet, where a controlled phase difference isachieved across the fiber cross-section over a predefined pattern. Theoptical fiber can be a single mode fiber or a multi-mode fiber. Either abinary or a complex phase difference can be achieved. Also disclosed isthe related system for shaping an output light beam from an opticalfiber.

The present invention provides a method of controlling the amplitude andphase of the output beam from an optical fiber. The purpose of thisinvention is to shape the output beam from an optical fiber in terms ofphase and amplitude using surface relief structures integrated directlyinto the fiber facet. Direct modification of the fiber end allows forcontrol of amplitude, phase and direction of the light beam profileexiting the optical fiber with direct implications in laser processing,optical trapping, super high-resolution fluorescence microscopy, opticalswitching etc.

The present invention allows for optical performance across a very broadwavelength range and across a wide range of materials. It provides for acheap implementation requiring, for example, a single master with thenegative of the structure of interest. That master can then be used tocreate the desired surface structure in multiple fibers without loss ofquality from one fiber to another. The direct alternative technique tothe method of the present invention is the use of external phase masks.However, these add to the complexity and the cost of the technique whilereducing the ruggedness.

These and other features and advantages of the invention, as well as theinvention itself, will become better understood by reference to thefollowing detailed description, appended claims, and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a π phase change created over the central portion of thebeam up to the 1·e−2 intensity boundary. Output field intensity shown isafter Fourier transform.

FIG. 2 shows a π phase change created across 50% of the beam. Outputfield intensity shown is after Fourier transform.

FIG. 3 shows that two types of gratings (2D linear and circular) on thefiber end facet will yield different light output profiles.

FIG. 4 shows a 2D linear grating stamped on the end face of a 22 μm corefiber of a low mode count As₂S₃ fiber (6 modes at wavelength of 4.8 μm).

FIG. 5 is an illustration of a surface structure that creates a 2π phasechange along the cross-section of the fiber end in a total of 8 steps.

FIG. 6 is an example of a commercially-available substrate with asurface structure that creates a 2π phase change along the cross-sectionof a laser beam.

FIG. 7 is an example of a multi-mode chalcogenide fiber stamped with a2D pattern.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a novel method and system for beamshaping through the modification of the near field directly at the exitfacet of the optical fiber. This is achieved through direct surfacetexturing of the fiber facet, which allows for controlled phase changeacross the beam diameter. This approach is very different from othermethods because it does not require an extraneous material to beattached or deposited to the fiber end, which makes the present approachmore robust and simpler to implement.

This type of surface texturing requires in certain situationsnanometer-level control of the fiber facet structures, as will be madeclear in the examples. The phase change will provide the required nearfield transformation without the need of external phase plates, therebyreducing system complexity and enhancing ruggedness. The surfacetexturing can be performed by stamping the fiber end onto typicalsubstrates such as silicon wafers or fused silica plates which have theappropriate patterns built in. US Patent Publication 20110033156 (2010)discloses a technique for surface microstructuring of optical fiber endswith intent of reducing the reflection loss occurring at the fiber-airinterface.

In one embodiment, the facet of a single-mode chalcogenide, fluoride,silica, silicate, germanate, tellurite or any other optical fiber ismodified such that a certain binary phase difference can be achieved ina controlled manner across the fiber cross-section over a predefinedpattern.

EXAMPLE 1

A single-mode fiber end-surface is modified with a circular step ofdepth d in the core region. The width of the step should match a certainportion of the diameter of the output beam. The depth of the step isdetermined by the desired phase change and the operating wavelength λ.

For single-mode fibers we require a π phase change in the centralportion of the beam, up to the 1·e⁻² (13.5%) diameter of the beam, withrespect to the remaining beam. The output beam is converted to a sincfunction whose Fourier transform (as given by lens or in the far field)is a flat top profile. The situation is illustrated in FIG. 1.

For this case, the required depth (d) of the surface relief is given byEquation (1), where n is the effective refractive index of the mode:

$\begin{matrix}{d = \frac{\lambda}{2\left( {n - 1} \right)}} & (1)\end{matrix}$

In particular, consider a typical single-mode As₂S₃ fiber with a 1·e⁻²diameter of about 6 μm and a cladding size of 170 μm. The effectiveindex of the fundamental mode is n=2.404 as determined from fiber Bragggratings data (Florea et al., “Fiber Bragg gratings in As₂S₃ fibersobtained using a 0/−1 phase mask,” Opt. Mat., 31, 942-944 (2009), theentire contents of which is incorporated herein by reference). Foroperation at λ=1.55 μm, one needs a surface relief depth d=552 nm. Otherchalcogenides can also be considered, such as As₂Se₃, with the operatingwavelength changed to accommodate the transmission window of thematerial.

EXAMPLE 2

Another particular case is that of a modified fiber end-surface wherehalf of the beam output aperture experiences a it phase shift withrespect to the other half, as illustrated in FIG. 2. The depth of thestep on the fiber surface is given by Equation (1) as well.

In another embodiment, the facet of a single-mode chalcogenide,fluoride, silica, silicate, germanate, tellurite or any other opticalfiber is modified such that a certain complex (non binary) phasedifference can be achieved in a controlled manner across the fibercross-section over a predefined pattern.

EXAMPLE 3

A single-mode fiber end-surface is modified with a grating of period Lin the core region (FIG. 3). A variety of gratings (circular, blazedetc.) are possible. The type, period and depth of the grating should beadjusted to provide the desired diffraction for the light beam exitingthe fiber. A variety of gratings and situations can be considered suchas to manipulate the amplitude and direction of the resulting outputbeams. FIG. 4 shows a 2D linear grating stamped on the end face of a 22μm core fiber of a low mode count As₂S₃ fiber (6 modes at wavelength of4.8 μm).

EXAMPLE 4

A 2π phase change is achieved by a finite number of steps created in aspiral pattern across the fiber end facet, around the center of thecross-section. This surface structure will create an output beam in theshape of a ring or doughnut, with no light in the center. The situationwhere the 2π phase change is created by a total of 8 steps isillustrated in FIG. 5.

The thickness of each step is easily calculated from the requirementthat the phase change occurring at each step be exactly 2π/8 and it isgiven by Equation (2):

$\begin{matrix}{d = \frac{\lambda}{8\left( {n - 1} \right)}} & (2)\end{matrix}$

In the case of a typical single-mode As₂S₃ fiber with an effective indexof the fundamental mode of n=2.404 and for operation at λ=1.55 μm oneneeds a step thickness d=138 nm. The control of the thickness isimportant but easily implemented given the advanced state of art of thefabrication techniques involved.

An extension of Example 4 is that of a spiral that has a very largenumber of steps or that achieves the 2π phase change in a continuousfashion rather than step-wise fashion. This surface structure will alsocreate an output beam in the shape of a ring or doughnut, with no lightin the center. This is essentially similar to a vortex phase plate,which is commercially available and which is illustrated in FIG. 6.

In another embodiment, the facet of a multi-mode chalcogenide, fluoride,silica, silicate, germanate, tellurite or any other optical fiber ismodified such that a certain binary phase difference can be achieved ina controlled manner across the fiber cross-section over a predefinedpattern. Of great interest is the situation of low-mode number fiberswhere phase change can be used as a modal filter.

EXAMPLE 5

A multimode As₂S₃ fiber has been stamped with a macroscopic 2D array ofholes and imaged in reflection mode with white light as shown in FIG. 7.

In another embodiment, the facet of a multi-mode chalcogenide, fluoride,silica, silicate, germanate, tellurite or any other optical fiber ismodified such that a certain complex (non binary) phase difference canbe achieved in a controlled manner across the fiber cross-section over apredefined pattern. Of interest is the situation of low-mode numberfibers where phase change can be used as a modal filter.

In another embodiment, the facet of a solid-core photonic crystal fiberis modified such that a certain binary phase difference can be achievedin a controlled manner across the fiber cross-section over a predefinedpattern. The photonic crystal fiber can be made of chalcogenide,fluoride, silica, silicate, germanate, tellurite or any suitablematerial.

In another embodiment, the facet of a solid-core photonic crystal fiberis modified such that a certain complex (non binary) phase differencecan be achieved in a controlled manner across the fiber cross-sectionover a predefined pattern. The photonic crystal fiber can be made ofchalcogenide, fluoride, silica, silicate, germanate, tellurite or anysuitable material.

The above descriptions are those of the preferred embodiments of theinvention. Various modifications and variations are possible in light ofthe above teachings without departing from the spirit and broaderaspects of the invention. It is therefore to be understood that theclaimed invention may be practiced otherwise than as specificallydescribed. Any references to claim elements in the singular, forexample, using the articles “a,” “an,” “the,” or “said,” is not to beconstrued as limiting the element to the singular.

What is claimed is:
 1. A system for shaping an output light beam, thesystem comprising: an optical fiber configured to transmit the outputlight beam; and an exit facet of the optical fiber, wherein the exitfacet comprises a surface textured according to a texturing patterndesigned to initiate a static phase difference between a first portionof the output light beam and a second portion of the output light beamsuch that the textured surface has a plurality of varying depths in theexit facet.
 2. The system of claim 1, wherein the optical fibercomprises chalcogenide, fluoride, or tellurite.
 3. The system of claim1, wherein the optical fiber comprises a solid core photonic crystalfiber.
 4. The system of claim 1, wherein the optical fiber is a singlemode fiber.
 5. The system of claim 1, wherein the optical fiber is amulti-mode fiber.
 6. The system of claim 1, wherein no additionalmaterial is attached to or deposited on the exit facet of the opticalfiber to initiate the static phase difference.
 7. The system of claim 1,wherein the surface is stamped to form the textured surface.
 8. Thesystem of claim 1, wherein the exit facet comprises a plurality of beamshaping elements formed by the textured surface.
 9. The system of claim1, wherein the textured surface comprises multiple steps created in aspiral pattern, and wherein the output light beam is in a shape of aring with no light in the center.
 10. The system of claim 1, wherein thetexturing pattern is a periodic texturing pattern.
 11. The system ofclaim 1, wherein the texturing pattern includes an array of circularsymmetric lines.
 12. The system of claim 1, wherein the texturingpattern includes an array of non-circular symmetric lines.
 13. A opticalfiber for shaping an output light beam transmitted through the opticalfiber, the optical fiber comprising: an exit facet; a first step formedon a surface of a first portion of the exit facet according to atexturing pattern designed to initiate a static phase difference betweena first portion of the output light beam and a second portion of theoutput light beam; and a second step formed on the surface of a secondportion of the exit facet according to the texturing pattern, wherein afirst depth of the first step in the surface of the exit facet isdifferent than a second depth of the second step in the surface of theexit facet.
 14. The optical fiber of claim 13, wherein the static phasedifference between the first portion of the output light beam and thesecond portion of the output light beam is a π phase shift.
 15. Theoptical fiber of claim 14, wherein a difference between the first depthand the second depth is determined according to the equationd=λ/(2(n−1)), wherein n represents an effective index of the fundamentalmode, wherein λ represents an operating wavelength, and wherein drepresents the difference between the first depth and the second depth.16. The optical fiber of claim 13, further comprising: a plurality ofsteps, including the first step and the second step, formed in a spiralpattern on the surface of the exit facet according to the texturingpattern, wherein the output light beam is in the shape of a ring with nolight in the center.
 17. The optical fiber of claim 13, wherein thetexturing pattern is a periodic texturing pattern.
 18. A optical fiberfor shaping an output light beam transmitted through the optical fiber,the optical fiber comprising: an exit facet; a first step formed on asurface of a first portion of the exit facet according to a texturingpattern designed to initiate a static phase difference between a firstportion of the output light beam and a second portion of the outputlight beam; and a second step formed on the surface of a second portionof the exit facet according to the texturing pattern, wherein a firstdepth of the first step in the surface of the exit facet is differentthan a second depth of the second step in the surface of the exit facet,and wherein the first step and the second step are formed such that,after passing through the exit facet, the first portion of the outputlight beam and the second portion of the output light beam differ inphase according to the static phase difference.
 19. The optical fiber ofclaim 18, wherein a light beam with a substantially uniform phase istransmitted through an entire core of the optical fiber to produce theoutput light beam.
 20. The optical fiber of claim 18, furthercomprising: a plurality of steps, including the first step and thesecond step, formed in a spiral pattern on the surface of the exit facetaccording to the texturing pattern, wherein the output light beam is inthe shape of a ring with no light in the center.